This invention relates to an ignition timing control system for an internal combustion engine, which comprises a function for limiting the ignition timing to within a proper range so as to ensure occurrence of ignition at an instant lying within the proper range of ignition timing, thereby preventing mis-ignition or ignition at an excessively advanced or retarded instant tending to result when the means for arithmetically calculating the ignition timing makes miscalculation or when the ignition timing is retarded in a transient condition of engine operation.
Ignition timing control systems of very complex structure capable of more accurate control of ignition timing than hiherto are now demanded to deal with the severe regulations on the engine exhaust gases, and also, to improve the fuel consumption of internal combustion engines. In an effort to meet the above demand, researches and studies are now directed to electronic ignition timing control systems which provide a higher freedom of ignition timing control than the conventional mechanical ignition timing control systems. Among various kinds of electronic ignition timing control systems proposed hitherto, an electronic ignition timing control system of the kind employing a microcomputer or microprocessor capable of arithmetically calculating the ignition timing for the complex control of the ignition timing is proved to be predominant, and reserches and studies are being directed to the production of practically usable model of such an electronic ignition timing control system.
However, various problems remain still to be solved before the electronic ignition timing control system of this kind can be put into practical use. The reliability is the most important problem among them. That is, although the electronic ignition timing control system of the kind employing the microcomputer or microprocessor can attain the desired complex control of the ignition timing in compliance with the operating requirement of the engine, it has such a drawback that mis-ignition tends to occur in a transient condition of engine operation such as an abrupt increase or decrease in the rotational speed of the engine, or ignition tends to occur at an excessively advanced or retarded instant. This is because misreading of input data or miscalculation of input data leads to an erroneous result of arithmetic calculation of ignition timing which will provide an inexactly timed output signal for the control of the primary winding of the ignition coil. Further, various sensors provided for sensing the operating conditions of the engine, for example, an intake manifold vacuum sensor, an intake air flow sensor and others will not be readily responsive to such variables but will respond more or less with a delay time. In such cases, the operability of the engine will be greatly impaired, the catalyst will be deteriorated due to the mis-ignition, and the engine will be forced to stop during running.